The Carriage Gates through which Khalid Masood entered the grounds of parliament.
Photograph: Jack Taylor/Getty Images

Demands are growing for security around the houses of parliament to be enhanced as police seek to establish whether the terrorist who attacked Westminster had scouted the area for weak points.

Video footage has emerged showing an entrance to the estate was left open in the immediate aftermath of Wednesday’s attack, in which five people including the attacker were killed and more than 50 injured.

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The footage shows the Carriage Gates, through which Khalid Masood entered the grounds of Westminster Palace where he stabbed PC Keith Palmer to death, left unguarded for several minutes.

As armed officers swarm the cobbled forecourt of New Palace Yard after shooting Masood, the iron gate through which vehicles enter the yard is left wide open. No police officers are visible guarding the gates, pedestrians walk past and a courier on a moped appears to enter unchallenged.

The video, obtained by the Times (£), fuelled concern that the attack could have been worse if Masood had had accomplices. Speaking on the BBC Radio 4 Today programme, the Conservative MP Nigel Evans said: “I’ve got no doubts whatsoever that there will be enhanced features of security, it’s happening on a regular basis, but following this tragedy, security has got to be upped at the same time as still having a welcoming hand to members of the public to come and see how democracy works.”

Evans said many MPs who were locked in the Commons chamber during and after the attack were discussing how to boost security in certain areas.

He said, however, that far more checks were carried out at Carriage Gates than at the time he was elected in 1992, when an officer would just “wave you through”.

“I’ve got no doubts whatsoever that, under the speaker, that he is regularly in touch with the Metropolitan police, and that they will be talking about now, new security measures that will be there,” Evans said.

Detectives were last night scouring large amounts of CCTV footage to see if Masood had been recorded scouting the Westminster area before his attack.

The Guardian understands they are also using camera records and automatic number plate recognition technology to see if Masood was captured driving his rental car, a Hyundai four-wheel drive, around Westminster Bridge to calculate the best location to mount the kerb and inflict the most damage.

At the same time, it is expected they will be looking for anyone else doing a similar thing to aid the attack, since which there have been 11 arrests and a series of vehicle seizures across the UK.

The former Met commissioner Ian Blair told Today he was “absolutely certain that there will be a review now of the outer soft ring that those of us who work at the Palace of Westminster are very used to”.

“Always behind it is the inner core of armed officers, but PC Keith Palmer has paid with his life for that soft outer rim and I think that his family at least, and everybody else, needs the reassurance that will be reviewed.”

Lord Blair refused to criticise the officers who apparently left the gate unguarded, describing them as “human beings” who would have been “gripped completely” by the attack, while knowing armed officers were on their way.

But he added: “I’m absolutely certain that there will have to be changes. People are used to the fact that if they go into Downing Street they are confronted by basically closed gates and armed officers and I’m afraid that’s what will have to happen, but we’ll leave it to the reviews to see what it is. But I don’t think there should be shock, horror about the fact the gates were open for a moment after that kind of attack.”

Parliamentary authorities have previously been warned about the potential vulnerability of Carriage Gates. Around 5,000 people work on the estate and about 14,000 people hold parliamentary passes, while hundreds of visitors enter the building each day.

On Friday, Mark Rowley, Scotland Yard’s anti-terror chief, said current arrangements were proportionate and that procedures for guarding parliament had been designed so they were not “overly intrusive”.

“Our current arrangements have been developed with parliament over many years and are designed to provide access to the seat of our government balanced with security that is proportionate but not overly intrusive,” he said.

The prime minister also defended security arrangements at parliament on Thursday, saying the UK should be clear that the attacker was unsuccessful in gaining access to the building.

The security expert Prof Anthony Glees told the Guardian the attack had demonstrated a “lack of protection” within parliament, while there were “disturbing signs of chaos” in the way May was taken away from the estate.

Counter-terrorism detectives have been investigating Masood’s life and associations across Britain in a race to discover what led him to run down pedestrians on Westminster Bridge, crash his vehicle outside parliament and attack two police officers.

A joint investigation by Scotland Yard’s counter-terrorism command and MI5 led to three further arrests on Thursday and early Friday in London, Birmingham and Manchester, with two of them still in custody. Properties were searched in Wales and southern England. It remains unclear whether the attack was carried out alone or with support or encouragement.

Fears have heightened that Masood was either radicalised overseas or in prison. The Saudi embassy in London confirmed late on Friday that Masood had been in the country three times and taught English there, from November 2005 to November 2006, from April 2008 to April 2009, and for six days in March 2015. He had a work visa and was not tracked by Saudi security services because he didn’t have a criminal record in the country.

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But the 52-year-old, born Adrian Elms, was known at home for having a violent temper and had criminal convictions in the UK. A childhood friend told The Sun newspaper Masood had converted to Islam while in jail.

“When he first came out he told me he’d become a Muslim in prison and I thought he was joking,” Mark Ashdown said. “Then I saw he was quieter and much more serious. I gave him some cash-in-hand work for a few months as a labourer.

“He said he needed time to pray and read the Qur’an – something about finding inner peace. There were still flashes of the old Ade, but they were few and far between.

“I heard he’d split from his partner and got even more deeply into religion. But nothing could have prepared me for hearing his name on the radio.”

Detectives have so far seized 2,700 items as part of the investigation, including “massive amounts” of computer data, while they have had contact with around 3,500 witnesses. Searches at three addresses are continuing.